Eight out of ten women take some form of medication during pregnancy despite advice to the contrary, research finds.
Drugs can affect the developing foetus
Doctors and pharmacists advise all drugs should be avoided in the first three months of pregnancy unless benefits outweigh risks to the foetus.
But a team from the University of Bristol found only 17% of mothers did not take any conventional medicine throughout their pregnancy.
The results are based on a survey of 14,000 pregnant women.
It is part of the major Children of the 90s project which has been following the progress of children and their parents in minute detail since the early 1990s.
Researcher Dr Judith Headley said: "Fortunately, relatively few drugs have been proven to cause malformations of the embryo or foetus.
"However, little is known about more subtle effects on foetal or child development."
A previous study from Children of the 90s showed a possible link between high usage of paracetamol during pregnancy and wheezing in early childhood - suggesting a connection to asthma.
The latest study found that 39% of women took analgesics - mostly paracetamol but also aspirin - during the early stages of pregnancy.
One in four took indigestion medicines, known as antacids, in mid to late pregnancy.
Dr Headley said it was not surprising, since many pregnant women will suffer from a variety of symptoms such as indigestion, nausea, vomiting, and haemorrhoids.
"Some conditions are treated with prescription drugs after assessment of risks and benefits with a doctor.
"However, minor ailments are often self-medicated, with over-the-counter products.
"Some women are also turning to alternative therapies to avoid taking conventional medication and may end up taking non-standardised herbal preparations which have not undergone the rigorous testing of more conventional products."
The advice given by the British National Formulary includes the statement: "Drugs should be prescribed in pregnancy only if the expected benefit to the mother is thought to be greater than the risk to the foetus, and all drugs should be avoided if possible in the first trimester."
Dr Headley said: "In this study there is little evidence of drug avoidance in the first trimester, although many of the products may have been self-administered rather than given on the advice of a doctor.
"All the research appears to suggest that the reported incidence of drug use in pregnancy is higher in recent years.
"It is a long time now since the world was shocked by the effects of thalidomide, so perhaps it is time to remind women who may become pregnant that some drugs can be harmful and that they should seek advice from a health professional before self-medicating."
A spokesperson for the British National Formulary said women should consult a doctor or pharmacist before taking any medicine or herbal remedies during pregnancy.
It was important to remember that some maternal illnesses, if unchecked, could be harmful to the foetus and in such cases taking a medicine might be a safer option.
Relatively few medicines had been associated with harm, and it was important not to panic pregnant women by over-stating the risks.
However, the spokesperson added: "The BNF advice that medicines should be avoided during pregnancy as far as possible is a very wise precaution."
"But before we get too concerned about the level of medicine taking during pregnancy, we need to have a clearer idea whether the outcome of the pregnancy in women who have taken medicines is statistically different from those who have not."
Professor David Haslam, chair of the Royal College of GPs, said: "Most pregnant women are aware that they should avoid prescription drugs unless they are absolutely essential, but fail to realise that over-the-counter products can also be harmful.
"The simple rule is that pregnant women should not take any treatments unless they are certain they are safe, or the benefits really do outweigh the risks. If in doubt talk to your pharmacist, doctor, or midwife."